It is possible, I'd say quite easily too.
There is a guy in the states who has reverse engineered the OE ecu to accomodate various outputs using the free'd up solenoids that no longer in use. Using one of these outputs you can activate the water injection by throttle %/rpm, etc.
But obviously for a given setup you would rather spend money in the head flowing before trying to play with this, unless your turbo where the gains are straigthforward.